Glass composites containing carbon nanotubes: Processing, characterisation and prospects for toughening

Carbon nanotubes (CNTs) are one of the most intensively studied nanostructured materials due to their remarkable electrical, thermal and mechanical properties. Such a combination of properties has ranked CNTs as a potentially attractive reinforcement in a variety of composite materials. However, unlike polymers and metals, comparatively little research has been focussed in ceramic matrices, particularly glass, although the subject has recently been reviewed. The effect of the diameter and aspect ratio of CNTs on the mechanical properties of such brittle systems is a research area to be explored. A fully dense amorphous silica matrix with uniform dispersion of CNTs has been produced to exhibit the true effect of CNTs as reinforcing filler in brittle matrices. Dense silica glass matrix composites reinforced with multi-walled carbon nanotubes (MWCNTs) were produced by colloidal processing by heterocoagulation. By controlling the surface chemistry, the quality of the diphasic suspension was maintained during mixing, ensuring electrostatically-driven coating of silica nanoparticles onto the MWCNTs. The heterocoagulation process produced isolated CNTs at loadings up to 15 wt.%. The colloidal processing was followed by either pressureless sintering or spark plasma sintering for densification of the composite to the theoretical density whilst minimising devitrification of the silica glass matrix material. As-produced composites with MWCNTs of different diameter / aspect ratio and varied loadings were characterised microscopically and mechanically. In addition to SEM, XRD, hardness and indentation fracture toughness, the composites were also characterized for thermal and electrical conductivities. One of the key objectives of this project was to investigate the real effect of CNTs on the fracture toughness, K1c of the CNT/silica composites. Therefore, detailed K1c measurements using both the indentation and Chevron notch tests were compared. This study provides insight into the continuing debate in the literature on the appropriateness of different fracture toughness measurements as applied to CNT/inorganic matrix composites. In addition, the possible toughening mechanisms associated with CNT pull-out, CNT bridging, and crack deflection were investigated